The present invention relates to a collapsible filter basket adapted to be introduced into a blood vessel of a patient, and comprising a plurality of resilient wires interconnected at their respective ends.
The invention is particularly, but not exclusively, concerned with vena cava filters which are devices introduced into the vena cava, preferably the inferior vena cava, to entrap thrombi or emboli in the blood flowing through the vein and prevent them from reaching the patient's lungs and causing pulmonary embolization. The specification of U.S. Pat. No. 3,952,747 to Kimmell, issued Apr. 27, 1976 and incorporated by reference herein, contains a rather compreshensive analysis of the background and the state of the art relating to vena cava filters at that time.
More recently there has been described in the medical literature a vena cava filter made of a thermal shape memory alloy called nitinol. In its expanded state, in which it is first formed at an elevated temperature, and which it assumes after being positioned in the vena cava by means of a catheter, the filter is shaped as a rather flat, umbrella-like cone in which emboli-capturing meshes are defined by crossing loops formed by the individual wires. The filter may be delivered, after being refrigerated and straightened, through a comparatively narrow catheter, and when positioned in the vena cava and subjected to the body temperature it will expand to its operative shape as described above.
Although in principle the delivery of the nitinol filter is rather uncomplicated there are some problems associated with the filter. It is necessary to keep the filter in iced saline prior to its delivery, and during the delivery operation the catheter has to be flushed continuously with iced saline. If the filter is not optimally positioned or oriented in the first instance, a correction is hardly possible because at body temperature the wire material does not possess the pliability which would be needed to permit the expanded filter to be straightened and pulled back into the catheter.